DISTRIBUTION OF BUTTERFLIES IN THE NEW WORLD vS61 



appear to be the most significant in controlling the distribution of 

 butterflies. 



Because they are dependent on the heat of the environment for 

 their activity, butterflies become inactive at temperatures below 

 freezing. The tropical environment therefore appears to be ideal. 

 This is reflected by the facts that 262 of the 311 genera (and species 

 groups) are equatorial and that 142 of these occupy the whole, or 

 almost the whole, distance from the Tropic of Cancer to the Tropic 

 of Capricorn. Adaptation to existence in northern or southern 

 latitudes with seasonal cold weather has been attained by members 

 of 7 out of 9 of the families in one of two ways: (1) by remaining 

 during the cold time of the year in an inactive state (diapause), or 

 (2) by migrating out of the area. The former method has been em- 

 ployed most successfully by members of 6 families, which thereby 

 are enabled to exist in areas of freezing winters. The members of the 

 seventh family (Danaidae), without diapause, avoid the hazard 

 of cold by migrating into warmer regions each winter. In fact, it is 

 not unusual for many species of several families to migrate south- 

 ward in winter and northward in summer, especially in areas on the 

 fringes of their ranges. 



Most species appear to have a limited range of temperature toler- 

 ance. This is obviously narrower for species than for genera, but 

 even for most genera it appears from the charted distribution to be 

 narrower. Genera adapted for cold climates seem to be unable to 

 exist in areas of continuous high temperatures. Their ranges typically 

 rise in altitude as they progress southward in the northern latitudes, 

 or northward in the southern latitudes. Even the distribution of 

 genera appears to be partly controlled by temperature tolerances. For 

 example, Colias is cold-adapted and skips over the tropics and sub- 

 tropics, whereas Zerene is adapted to subtropical conditions and 

 remains below the cold regions (altitudinally and latitudinally) and 

 above the hot temperatures (Fig. 4). Other genera occupy the hot 

 areas. The species within a genus occupy belts of latitudinal diversi- 

 fication. The species of Colias can be arranged in consecutive order 

 from those most cold-adapted to those most warm-adapted. The 

 most warm-adapted species {Colias enry theme) lacks a diapause and 

 depends for its survival in the winter either on migration or on 

 resistance to the relatively short winters in the temperate zone. It 

 cannot survive in the far north. It migrates northward in the Mis- 



